The present invention relates to a fabricating machine in which a machining tool or a machining electrode intrudes into a workpiece and is subsequently moved relative to the workpiece in a direction substantially perpendicular to the tool or electrode machining direction. More particularly, the invention relates to improvements in a discharge machining method and apparatus in which a workpiece is machined by electric discharge which is carried out between the workpiece and the machining electrode while the electrode is moved relative to the workpiece in a coarse, primary or main machining direction and is subsequently finish machined while the electrode is moved relative to the workpiece substantially in a direction perpendicular to the main machining direction and accordingly in a circular motion.
In a conventional fabricating machine such as, for example, discharge machining method and apparatus, a workpiece is machined by electric discharge which is carried out between the workpiece and the machining electrode while the electrode intrudes into or is moved relative to the workpiece in a coarse, primary or main machining direction and is normally controlled by a servo so that the electrode is maintained at a constant distance from the workpiece in the same machining direction so as to machine the workpiece. In an ordinary conventional discharge machining operation, after the workpiece is coarsely or primarily machined, it is successively finish machined with a plurality of machining electrodes of similar shape or configuration to the previous electrode but of a slightly different size. This is because the machining speed is high during the coarse or primary machining operation but the surface of the workpiece to be machined is coarse, while the machining speed is low during the finish machining operation but the surface of the workpiece to be finished is fine and yet the machining side gap between the electrode and the workpiece should be narrow in the finish machining operation.
Accordingly, there have been proposed discharge machining method and apparatus as described below for the purpose of both coarsely and finish machining a workpiece with only one machining electrode. That is, in such discharge machining, after a workpiece is coarsely or primarily machined, a secondary machining feed in a direction substantially perpendicular to the coarse or primary machining direction, for example, a circular motion, is imparted to the electrode or the workpiece, and the workpiece is finish machined with the same machining electrode as that used in the coarse or primary machining operation similarly to the effect that a machining electrode which has a larger apparent size is employed. One example of such discharge machining apparatus shown in FIG. 1.
In FIG. 1, a machining electrode 10 confronts a workpiece 12 in a machining insulating solution. A pulse current supplied from a pulse current supply source 14 is fed through a machining gap between the electrode and the workpiece, and the workpiece 12 is thereby machined by an electric discharge therebetween. At this time a voltage Vd corresponding to the voltage across the machining gap and a reference voltage Vs are applied to a voltage differential circuit 16, which in turn produces a differential voltage output Vd-Vs. This voltage output Vd-Vs is sequentially applied to an amplifier 18, which amplifies the differential voltage Vd-Vs from the differential circuit 16 and produces an amplified differential voltage Vd-Vs to the hydraulic servo valve 20, thereby controlling the servo valve 20 in accordance with the differential voltage Vd-Vs. Thus, a hydraulic cylinder 22 is controlled by the servo valve 20 and in turn controls the electrode 10 mounted through X-Y intersection tables 30 and 32 and a piston rod in a servo mechanism so that the voltage Vd may coincide with the reference voltage Vs and accordingly the electrode 10 is moved relative to the workpiece 12 in a main machining direction or in Z-axis direction. The voltage differential circuit 16 and the amplifier 18 form a servo circuit, and the hydraulic servo valve 20 and the hydraulic cylinder 22 form a servo mechanism.
After the workpiece 12 is machined coarsely or primarily to the depth set slightly before the final predetermined depth by the electrode 10 and the coarse or primary machining step is thus finished, the pulse current supply source 14 is switched to produce a pulse output having smaller energy per pulse. Further, an electrode movement control device 24 produces output signals to both servo motors 26 and 28 in an ordinary method. The servo motors 26 and 28 thus operated move the X-Y intersection tables 30 and 32 in X-axis and Y-axis directions, respectively, thereby imparting a circular motion and thus finish machining the workpiece with the electrode 10. In this case, to the servo motors 26 and 28 are applied sine wave voltages having different phases of .pi./2 and amplitudes corresponding to the side gaps between the electrode and the workpiece for the coarse or primary machining and the finish machining. Thus, the workpiece 12 is subsequently machined again in a desired predetermined depth while the electrode 10 is moved relative to the workpiece 12 in a circular motion. Since the electrode 10 is so moved relative to the workpiece 12 as to be in size corresponding to the diameter of the relative circular motion or with the diameter in size corresponding thereto as increased in an equivalent effect, the coarsely machined surface of the workpiece 12 in a primary step can be removed to be finish machined.
In the case where a workpiece 12 is machined to form a hole, as shown in FIG. 2, corresponding to a machining electrode 10 having an elliptical cross section shown with the electrode 10 in such discharge machining method and apparatus, the amount of the workpiece 10 to be removed at the portion of the locus of the electrode 10 moved in a circular motion is much larger than the portion having a smaller radius of curvature with the electrode 10 having a larger radius of curvature. Accordingly, as the workpiece 12 is proceeded in machining, the depth of the hole machined in the workpiece 12 of the portion having larger radius of curvature becomes much different as shown in FIG. 3 from the depth of the hole machined in the workpiece of the portion having smaller radius of curvature. Therefore, if a deep hole is machined in the workpiece by the discharge machining method and apparatus, the primarily coarsely machined surface cannot be sufficiently removed in the workpiece in the secondarily finish machining step, and the depth of the electrode 10 reaching in the workpiece in the finish machining becomes much different from the depth of the hole thus formed in accordance with the shape or configuration of the electrode as serious disadvantage.
There has been proposed, for the purpose of eliminating the aforementioned disadvantage of the conventional discharge machining method and apparatus, another discharge machining apparatus employing a following discharge machining method. FIG. 4 is an explanatory diagram of the principle of such discharge machining apparatus. It is evident that the cause of the disadvantage pointed out above in the discharge machining as indicated in FIG. 2 depends upon the difference in the machining margins in the X-Y plane with respect to the Z-axis. If the radii of the circular motion imparted to the X-Y intersection tables 30 and 32 are not initially set to the amounts corresponding to predetermined machining margins so as to solve the disadvantage but are gradually increased from zero, the electrode is initially disposed at the deepest position even in any deep machining and is moved in expanded manner from this state. As a consequence, the difference in the radius of curvature shown in FIG. 3 cannot occur. More specifically, the spiral circle indicated in FIG. 4 shows a diagram indicating the locus of the relative movement between the electrode 10 and the workpiece 12 according to this discharge machining method and illustrating the increased machining margin at every circular motion by .DELTA.R.
If the increased machining margin .DELTA.R is set to extremely small value, the machining extent at every circular motion of the machining electrode becomes extremely small so that the machining energy is sufficiently afforded for uniform machining of the workpiece with the electrode. However, such an allowance of the machining energy of the electrode in turn corresponds to the machining operation of the electrode for the workpiece with less machining capacity with the resultant long machining time and the decreased machining efficiency. If the machining extent .DELTA.R of the electrode is increased to prevent this drawback, the machining margin of the workpiece is increased with the resultant increased machining extent in the workpiece with more than the machining capacity of the electrode and accordingly causes disadvantageous difference in the depths in the workpiece as indicated in FIG. 3. Accordingly, it is always necessary to adjust the increased machining extent of the workpiece in response to the machining state of the electrode.
In such a discharge machining operation, machining chip and sludge exhausting action utilizing the increase and decrease in the machining gap between the electrode and the workpiece with the circular motion of the electrode relative to the workpiece is expected, and it is accordingly necessary to hasten the exhaust of machining chips and sludge by increasing the width in the increase of the machining gap between the electrode and the workpiece and accelerating the circular motion of the electrode relative to the workpiece when the machining state is deteriorated due to the shortcircuit or a like. However, if the circular motion of the electrode relative to the workpiece is further increased, the discharge machining apparatus does not follow a control system thereby deteriorating the machining accuracy of the electrode at the workpiece. This cannot allow simple increase of the circular motion of the machining electrode.
Further, in FIG. 4, the increase simply in the machining margin .DELTA.R in case of deep hole machining operation allows preferable machining with small machining margin, but causes, in case of large machining margin, a problem pointed out above with reference to FIG. 3 and induces a phenomenon that the electrode 10 is raised in the state to rub off the workpiece 12 while shortcircuit occurs between the electrode 10 and the workpiece 12.
Since the discharge machining electrode 10 is generally formed of a material being feasibly scratched such as copper or graphite, the electrode 10 is always scratched when the abovedescribed phenomenon occurs. Accordingly, it is preferred that, when the width of the machining gap between the electrode 10 and the workpiece 12 is narrow, the circulating speed of the electrode 10 is decelerated thereby securing a time while the electrode 10 may sufficiently escape upwardly in this case, while when the width of the machining gap therebetween is wide, the circulating speed of the electrode 10 is accelerated so as to readily exhaust the machining chips and sludge existing in the gap. It is necessary in the case where a shortcircuit or an abnormal electric discharge occur between the electrode 10 and the workpiece 12 to reduce the radius of the circulating electrode and to thereby accelerate the circulating speed of the electrode so as to exhaust the machining chips and sludge in the machining gap. However, the conventional discharge machining apparatus cannot control such a complicated operation.
Moreover, the worst drawback in the conventional discharge machining exists in the consumption of the machining electrode 10. FIGS. 5A through 5C illustrate examples of the drawbacks existing in the conventional discharge machining. If the degree of expanding the radius of the circular motion of the machining electrode 10 relative to the workpiece 12 is increased like in a deep hole machining operation, a shortcircuit will occur as indicated in FIG. 5A between the side surface of the electrode 10 and the workpiece 12. When such a shortcircuit heretofore occurs between the electrode 10 and the workpiece 12, the electrode 10 is raised as indicated in FIG. 5B along a direction of primary machining feeding and accordingly Z-axis direction, and after the shortcircuit state is eliminated, the electrode 10 is lowered as indicated in FIG. 5C while machining the workpiece 12. Since an electric discharge occurs at the end of the electrode 10 at this time, the machining operation is executed only at the end of the electrode 10, and the electrode 10 is accordingly locally consumed at the end thereof, with the result that the machining accuracy of the workpiece 12 becomes worse.